This application claims a priority based on German application 199 40 098.9 filed on Aug. 24, 1999, and the contents of that application are incorporated herein by reference.
The present invention concerns a process for controlling headlight range of a motor vehicle wherein signals are measured at front and rear axles representative of a position of a vehicle chassis of the motor vehicle relative to the front and rear axles and a signal corresponding to the speed of the motor vehicle is further generated, with the signals measured at the front and rear axles as well as the signal corresponding to the speed of the motor vehicle being further processed for allowing recognition of presence of a poor road surface. This invention also includes a device for carrying out this process.
When a motor vehicle with a headlight range control travels on a poor road surface, which is usually characterized by irregularly arranged unevenness, malfunctions of an automatic headlight range control may occur. There exists, for example, the possibility that, with sensors arranged on front and rear axles for determining height or inclination of the motor vehicle, shakes of a chassis could be interpreted as rotary motions around a pitch axis such that the device for headlight range control sets inappropriate headlight ranges; which could result, for example, in blinding opposing traffic.
A process, as well as a device for performing the process, of the type mentioned in the introductory paragraph above are disclosed in German patent publication, Offenlegungsschrift DE 196 53 662 A1. With the device described therein, vehicle height sensors are provided, for example, which detect changes in vehicle height near right front and left rear wheels of a vehicle. A terrain recognition unit according to the aforementioned German publication compares temporal courses of output signals of these two vehicle height sensors with each other, with the output signal of the vehicle height sensor at the rear axle being provided with a speed-dependent temporal offset in order to detect a correlation between the signals. The aforementioned device accurately makes a positive decision regarding recognition of a poor road surface when no correlation between the two signals thus compared with each other exists for a predetermined measurement period or when this correlation is relatively small. If the terrain recognition unit notes a clearly defined correlation between the two signals thus compared with each other, it decides that no poor road surface is present. In the event of a positive decision with regard to the presence of a poor road surface, the terrain recognition unit causes an adjustment device to adjust the headlight range to a certain predetermined value. After leaving the poor road surface, the headlight range control is again released.
It has proven disadvantageous with the aforementioned device and the process performed thereby that a relatively complicated comparison procedure with regard to the correlation of the signals of the front and a rear vehicle height sensors must be performed. Moreover, in this known device, the vehicle height sensors must be arranged on different sides of the vehicle.
It is an object of this invention to create a process and a device to control a headlight range of a motor vehicle of the type mentioned in the introduction above, which can be carried out in an uncomplicated manner and which has an uncomplicated structure, respectively.
According to principles of this invention, a difference between measured signals at a first point in time on a front axle and a rear axle is calculated and stored. Then, a difference between the measured signals at the front and rear axles is calculated at a second, later, point in time. Then, the two aforementioned differences are subtracted from each other, with a signal obtained being divided by a signal corresponding to a speed of the motor vehicle. If the signal obtained through this division exceeds a predetermined threshold value, a decision is made by a terrain recognition unit that a poor road surface is present. By means of the aforementioned calculations of differences, repeated at regular temporal interval, for example, and the subsequent division, the signals measured at the front and rear axles of the motor vehicle are evaluated in a simple and effective manner relative to vehicle speed.
When a front axle transmitter and a rear axle transmitter issue signals from vehicle height sensors, for example, the difference between the two signals delivers a statement concerning an inclination of the vehicle in a direction of travel, that is of a pitch angle. By calculating a difference between two of the differences between these signals taken at a predetermined time-interval spacing, the temporal change in the inclination of the vehicle is thus determined. By dividing with a signal corresponding to the speed of the vehicle, it is guaranteed that a positive decision with regard to presence of a poor road surface is made only, if even in slow travel, the inclination of the vehicle changes significantly in the direction of travel within a relatively short period of time. If, in contrast, the vehicle is moving at a high speed on a street which has continuous, but yet significant, changing road-bed inclinations in the direction of travel, such as is, for example, the case at a rounded top of a hill, no positive decision is made recognizing a poor road surface, so that the headlight range control remains in operation.
In an enhanced process, the signals measured, for example, on the front axle and on the rear axle by an enhanced device of this invention can in each case be multiplied by a characteristic factor which takes into account respective characteristics of the sensors detecting the signals. The difference between the signals measured at a first point in time on the front and rear axles and multiplied by the respective characteristics is calculated and stored. From this difference, a difference of signals measured on the front and rear axles at a second, later, point in time and multiplied by the respective characteristic factors can be calculated. As in the basic process, in this enhanced process as well, the signal obtained in the previous process step is divided by a signal corresponding to the speed of the motor vehicle and accordingly compared with a threshold value to decide whether a poor road surface is present. It has proven advantageous that in signals multiplied by the characteristic factor, the dependencies of the signals on the respective axle sensor kinematics have already been taken into account.
Advantageously, in the calculation of the difference of the signals multiplied by the respective characteristic factors, an offset value may be added such that the output signal thus obtained can be used as a desired-value signal for the headlight range control.
In the two aforementioned alternative embodiments of the process according to the invention, the process steps of difference calculation, division, and comparison with the predetermined threshold value may be repeated at regular or speed-dependent time intervals.
According to a preferred embodiment of the present invention, the possibility exists, in the presence of a positive decision relative to recognition of a poor road surface, to adjust the headlight range of the motor vehicle to a predetermined value, which preferably corresponds to a value in a normal position of the motor vehicle. Alternatively, in the presence of a positive decision relative to recognition of a poor road surface, the headlight range of the motor vehicle can be kept constant at the value existing at the time of the decision. With both processes, it is avoided that the headlight range is adjusted as a function of frequently changing inclination of the vehicle on a poor road surface.
According to a preferred embodiment of the present invention, the signal obtained upon division by the signal corresponding to the speed of the motor vehicle is filtered by a preferably digital lowpass filter before comparison with the predetermined threshold value. By use of this lowpass filter, high frequency components of the signal, which are generated, for example, by vibrations of the vehicle, are suppressed.
Advantageously, in the presence of a positive decision of recognition of a poor road surface, a counter is set to a predetermined initial value, whereby after the presence of a negative decision relative to the recognition of a poor road surface, when the value drops below the predetermined threshold value, the counter set to the predetermined initial value is counted, or resets downwardly at predetermined time intervals, with the headlight range control remaining deactivated as long as the counter has a value other than zero or greater than a predetermined lower threshold value. The downward counting of the counter can preferably be designed to be speed-dependent such that with each counting procedure a value corresponding to the speed of the motor vehicle is subtracted from the counter value. Thus, after leaving the poor road surface, the headlight range control is switched on again after a period of time which is dependent on the speed of the motor vehicle. Thus, if the motor vehicle moves away from the poor road surface at a relatively high speed, the headlight range control is already available again after a short time such that the headlight range can be adapted to driving conditions.